Single fraction radiosurgery/stereotactic body radiation therapy (SBRT) for spine metastasis: A dosimetric comparison of multiple delivery platforms

Abstract There are numerous commercial radiotherapy systems capable of delivering single fraction spine radiosurgery/SBRT. We aim to compare the capabilities of several of these systems to deliver this treatment when following standardized criteria from a national protocol. Four distinct target lesions representing various case presentations of spine metastases were contoured in both the thoracic and lumbar spine of an anthropomorphic SBRT phantom. Single fraction radiosurgery/SBRT plans were designed for each target with each of our treatment platforms. Plans were prescribed to 16 Gy in one fraction to cover 90% of the target volume using normal tissue and target constraints from RTOG 0631. We analyzed these plans with priority on the dose to 10% of the partial spinal cord and dose to 0.03 cc of the spinal cord. Each system was able to maintain 90% target coverage while meeting all the constraints of RTOG 0631. On average, CyberKnife was able to achieve the lowest spinal cord doses overall and also generated the sharpest dose falloff as indicated by the Paddick gradient index. Treatment times varied widely depending on the modality utilized. On average, treatment can be delivered faster with Flattening Filter Free RapidArc and Tomotherapy, compared to Vero and Cyberknife. While all systems analyzed were able to meet the dose constraints of RTOG 0631, unique characteristics of individual treatment modalities may guide modality selection. Specifically, certain modalities performed better than the others for specific target shapes and locations, and delivery time varied significantly among the different modalities. These findings could provide guidance in determining which of the available modalities would be preferable for the treatment of spine metastases based on individualized treatment goals.

Spinal metastases are a common oncologic occurrence that can have a major impact on the cancer patient's quality of life and functional status. It is well known that radiation therapy is an excellent palliative treatment for spine metastases. Currently, accepted radiation techniques include a variety of fractionated regimens as well as single fraction treatment, which has historically been delivered at a dose of 8 Gy. Multiple studies have shown these techniques to result in a pain response of approximately 60%. 1,2 More recent data support the use of stereotactic body radiation therapy (SBRT) or radiosurgery for spinal metastases with fewer fractions delivered and greater, more durable responses. Gerszten et al. reported that 86% of patients experienced long-term pain improvement and excellent local control utilizing SBRT. 3 In current practice, an increasing percentage of patients with spine metastases can experience long-term survival. As systemic therapy continues to improve, it becomes even more important to produce durable pain palliation and local control. 4 SBRT is commonly defined as a treatment that couples a high degree of anatomic targeting accuracy and reproducibility with very high doses of extremely precise, externally generated, ionizing radiation delivered in five or fewer fractions to an extracranial target.
Treatment consisting of one fraction only is referred to as radiosurgery. The use of radiosurgery/SBRT has increased significantly over the last several years. A recent survey of radiation oncologists practicing in the United States reported that 63.9% use SBRT for selected patients, the most common treatment locations including lung, spine, and liver. 5 As utilization of this technique increases, so have the number of platforms designed to deliver such treatment. At our own institution, we have multiple treatment planning and delivery systems used for highly conformal SBRT treatments, but no set guidelines for choosing between them.
Previous reports on modality selection for SRS/SBRT have been published for intracranial sites and were either limited to two platforms 6 or compared based on technical specifications. 7 Within this study, we attempt to determine whether there are significant differences in planning and delivery capabilities across these platforms within the context of the current RTOG 0631 radiosurgery/SBRT spine trial. Therefore, we designed sample spine metastasis cases within a phantom model and generated radiosurgery treatment plans for five different planning and delivery systems. We hypothesized that, while each modality would be able to meet the constraints of RTOG 0631, there would be differences in dose to critical normal tissue, treatment time, and dose fall off that may assist in the choice of delivery system based on characteristics of the individual case and target shape/volume. The spinal cord was contoured as a structure approximately 7 mm in diameter, contained centrally within the bony limits of the spinal canal. The contoured cord was designed to reflect the average cord size of our previous ten radiosurgery spine patients who were planned using fusion of MRI or CT myelogram as well as measurements reported in the literature. 9 To ensure comparability, each contour and image set was communicated unaltered from Eclipse to each of the other treatment planning systems through DICOM-RT.

| METHODS AND MATERIALS
Dose objectives and constraints were designed to meet those required for RTOG 0631 and the target was prescribed 16 Gy in a single fraction. Briefly, planning requirements included the following: Axial representations of target volumes (Red) and spinal cord (Green) along with sagittal image of target "D" to illustrate its extent across two vertebral levels.
at least 90% of the target volume receives the prescribed radiosurgery dose; hotspots outside the target were limited to 105% within 1 cm of the target volume and 110% anywhere outside the target. Spinal cord constraints included 10 Gy to 10% of the partial spinal cord defined as 5-6 mm above and below the target, the total volume of spinal cord receiving 10 Gy was restricted to below 0.35 cc, and the absolute maximum dose to the spinal cord was restricted to 14 Gy to a volume of no more than 0.03 cc. Additional OAR constraints included cauda equina volume of < 0.03 cc receiving 16 Gy, and < 5 cc receiving 14 Gy. The total lung was limited to a volume of less than 1000 cc receiving 7.4 Gy. A point dose of 110% of the prescribed dose was allowed outside the target volume as long as it was less than 0.03 cc, which is an acceptable variation per the protocol.
With these objectives, single fraction radiosurgery plans were designed for each target to be delivered with CyberKnife (CK) with We analyzed these plans with priority on the dose to 10% of the partial spinal cord and dose to 0.03 cc of the spinal cord. The Paddick dose gradient index (PGI), defined as the ratio of the volume encompassed by half the prescription dose to the volume encompassed by the prescription dose, was used as a measure of the steepness of the dose gradient around the target. 10 Once we confirmed that each system was able to meet all of the target goals of the protocol, we compared these two cord metrics along with their ability to limit the dose to other surrounding tissues using the PGI.

| RESULTS
A total of 40 plans were generated for the cases listed above (eight for each platform-one for each lumbar and thoracic target) In an associated quality assurance analysis study, 11